A large segment of the population have chronic back and neck problems which cause pain and disability. The cause of the pain is often due to irritation of nerves, nerve roots or the spinal cord itself. This irritation can be caused by both chemical and mechanical processes. For example, nuclear material from the disc has been shown to irritate nerve cells. Also, pressure, resulting from contact with adjacent tissue can also be applied directly and induce pain in nerve tissues. A second cause of pain is believed to be degeneration of the disc material that lies between two adjacent vertebrae. Degeneration can result in pain directly, or it can lead to abnormal motions which result in pain.
Surgical techniques have been developed to eliminate the cause of the persistent pain either by clearing space around the nerves, nerve roots or spinal cord (so called decompressions) or by removing the disc and filling the space. The space filler can be a device that prevents motion (fusion) or can be a device that permits motion, preferably in a physiologic way. In some decompressive cases, it is necessary to remove part of the vertebral body and the adjacent discs.
Known prosthetic discs are typically designed to allow for some motion between the adjacent vertebrae. However, these known devices typically require resection of some limited amount of bone so as to obtain fixation. This is problematic for both the cases of true disc degeneration, and for many decompressive cases. In true disc degeneration, the disc is the source of pain and its removal is required, not the bone of the adjacent vertebral bodies. Removal of the bone, through a variety of instruments, can be surgically difficult and typically removes the densest cancellous bone in the vertebral body, thereby weakening the reconstruction. In decompressive cases where a portion of the vertebral body must be removed, the existing prosthetic devices are not designed to fit the void space created by the decompression. Devices that fit the space do not permit motion between the replaced vertebral body and adjacent vertebral bodies.
Accordingly, a spinal motion device is needed which permits either replacement of the disc without bone removal or that permits partial resection of the bone and still restores motion.